Conventionally, in a spark ignition type internal combustion engine that is installed in a vehicle, especially an automobile, a spark discharge between a central electrode and a ground electrode of an ignition plug is employed to ignite fuel air mixture in a combustion chamber at each ignition timing. In recent years, plasma generated by a microwave in combination with the spark discharge is employed so as to improve ignition efficiency of the spark discharge as disclosed in, for example, Japanese Unexamined Patent Application, Publication No. 2010-1827.
In the ignition device disclosed in Japanese Unexamined Patent Application, Publication No. 2010-1827, an antenna is arranged in the vicinity of the ignition plug so as to emit a microwave to the combustion chamber, thereby forming a plasma generation region. A magnetron is connected to the antenna so as to emit the microwave toward the vicinity of a discharge gap of the ignition plug, thereby forming the plasma generation region in the vicinity of the discharge gap.
With such configuration, fuel air mixture is ignited due to spark discharge occurred in the plasma generation region during the expansion stroke, and thereby enlarging a flame kernel. Furthermore, the flame kernel reacts with radicals in the plasma to promote combustion. However, with the ignition device disclosed in Japanese Unexamined Patent Application, Publication No. 2010-1827, the plasma generation region can be formed only in the vicinity of the ignition plug. As a result, a good combustion may be attained only in the vicinity of the ignition plug. This means that the propagation process of combustion to the entire combustion chamber is almost same to that of the conventional spark ignition type internal combustion engine in which plasma is not employed. Therefore, it has been difficult to expect the improvement of the fuel efficiency by the plasma.